Prospects for a proposed third runway at Hong Kong International Airport seemed uncertain yesterday as environment advisers delayed their decision on whether to approve its environmental impact assessment study.

The advisers - from a subcommittee under the Advisory Council on the Environment - were concerned about how adequate and effective measures to mitigate the project's impact on the threatened Chinese white dolphin habitat would be.

If the study is approved and the HK$130 billion project is given the go-ahead, some 650 hectares of prime habitat for the shrinking dolphin population would be lost to land reclamation for the third runway. Construction would last from 2016 to 2023.

The Airport Authority will respond in writing to further queries from the subcommittee, before another meeting on Monday for the advisers to deliberate their decision.

The subcommittee, which has spent 15 hours in three days grilling the authority's officials on the environmental impact assessment study, met yesterday afternoon to discuss whether to recommend the advisory council to endorse the report.

But by the end of the meeting, it had still not drawn a conclusion on the city's single most costly infrastructure project. The council has to submit its views by late next month to the environmental protection director, who will then decide whether to issue a work permit for the project.

A subcommittee member, who wanted to remain anonymous, said members at the meeting "freely expressed their opinions" about the report and what outstanding issues had to be further addressed by the authority.

"We haven't come to the time to indicate our preference," he said. "This takes time as … environmental impact assessment is a very complex issue."

Another member said the subcommittee had a number of doubts on the mitigation measures to protect the dolphins during construction and what could be done to draw them back after the work is done. The authority's replies had been unconvincing, he said.

The authority has so far agreed to set up a 2,400 hectare marine park to compensate for the habitat loss, but will build the park only after the runway is completed in 2023.

It also promised to re-route its Skypier high-speed ferry services and lower the ferries' speeds during construction, but rejected suggestions to relocate the pier from the east to the west side of the airport.

The authority's other mitigating measures include adopting a non-dredging reclamation method to reduce underwater noise that would affect the dolphins, and to set up an eco-enhancement fund to support dolphin research.

The subcommittee member said the group was also concerned about the authority's role as a proponent of the large-scale project that would involve various government departments.

"The authority can't speak for the government, and this leads to the question: to what extent does it have the power to do what it has pledged to do," he said.

Samuel Hung Ka-yiu, a dolphin expert who has been opposing the runway project, said he was pessimistic that the subcommittee would reject the controversial project.

"The government's hands are everywhere and officials will make sure that the project is passed," he said.